The key to making good onigiri is to have freshly cooked, hot rice. You can't make good onigiri with cold rice.

Wet your impeccably clean hands with cold water, and sprinkle them with salt. Take 1/4th of the rice and place on one hand. Make a dent in the middle of the rice with your other hand. Put in about 1 tsp or so worth of filling in the dent.

Working rapidly, wrap the rice around the filling, and form into a ball. To make the traditional triangular shape, cup your hand sharply to form each corner, and keep turning it until you are happy with the shape. Practive makes perfect.

Wrap the rice ball with 1-2 strips of nori seaweed.

Repeat for the rest of the rice.

The key to making good onigiri is to have freshly cooked, hot rice. You can't make good onigiri with cold rice.

Wet your impeccably clean hands with cold water, and sprinkle them with salt. Take 1/4th of the rice and place on one hand. Make a dent in the middle of the rice with your other hand. Put in about 1 tsp or so worth of filling in the dent.

Working rapidly, wrap the rice around the filling, and form into a ball. To make the traditional triangular shape, cup your hand sharply to form each corner, and keep turning it until you are happy with the shape. Practive makes perfect.

Wrap the rice ball with 1-2 strips of nori seaweed.

Repeat for the rest of the rice.

To bring along on picnic, wrap in plastic film or in a bamboo leaf (which is traditional). Some people prefer to carry the nori strips separately, and to wrap them around the onigiri when eating, to preserve the crisp texture of the seaweed.

If it's hard to get a hold of the traditional fillings, here are some non-traditional ones that I have tried that work well. However, unlike the more traditional fillings (especially umeboshi) these fillings are quite perishable, so be careful in hot weather if you are taking them for a picnic. Any rather strongly flavored, salty filling should work.

* Ground meat (pork or beef or a mixture), cooked with grated or chopped ginger, then flavored with soy sauce, some red pepper flakes, sake or mirin, and sugar. It should be quite dry. Curry flavored ground meat mixture works surprisingly well too.
* Canned tuna, well drained and flaked, flavored with a bit of soy sauce and/or salt to taste.
* Flaked corned beef
* Chopped up western style pickles (as long as they don't have too much garlic in the brine), well squeezed to get rid of excessive moisture

I'd like to say that for beginners, it's best to make the kind of onigiri that is wrapped completely. I just cut it into a square a little bit bigger than 2 times the onigiri size. Then I wrap it on both sides and fold down the corners. Practice makes perfect though!!